Macrolides are a class of antibiotics of antibacterial activity against gram-positive bacteria, gram-negative cocci, and mycoplasmas. The erythromycins are among the most important macrolide antibiotics and used extensively in human medicine. With a collaboration with Abbott Laboratories, we have initiated a project of exploring the possibility of applying various solid-state NMR techniques developed from this laboratory to the study of macrolide antibiotics. In particular, we intend to apply solid-state NMR in studying the conformation of erythromycin antibiotics and its binding to the 50-S ribosome subunits. We have obtained solid-state magic-angle-spinning (MAS) 13C NMR spectra of erythromycin A phosphate at 79 MHz. The 13C MAS NMR spectra exhibit very high resolution, confirming that our initial proposal for various solid-state NMR experiments is valid. Interestingly, the 13C MAS NMR spectra also suggested that two different crystallographic forms (approximately 1:1) are present in this particular sample. Since neither solid-state 13C NMR spectra nor crystal structure data of this compound are available in the literature, it is unclear at the present time as to what are the structural differences between the two crystallographically distinct forms. However, we plan to recrystallize the sample from methylene chloride and remeasure 13C MAS NMR spectra. It is clear that a sample consisting of only one crystallographic form will yield much better 13C MAS NMR spectra, especially in the region from 0 to 80 ppm. This improvement in resolution may prove crucial for some future solid-state 13C NMR measurements. In addition, we plan to investigate the field-dependence of the line-widths observed in 13C MAS NMR spectra of this erythromycin derivative. In particular, we plan to obtain 13C MAS NMR spectra of the aforementioned sample at a lower field, 4.7 T (50 MHz for 13C nuclei). It is a general observation that line-widths in 13C MAS NMR spectra decrease as the applied magnetic field is decreased. In summary, all these studies on erythromycin A phosphate will be very useful for further solid-state 13C NMR studies of 13C-labeled erythromycin compounds and their complexes to the ribosome.